Question

Give one chemical test to distinguish between the following pair of compounds.

Aniline and benzylamine

Answer 1

Aniline and benzylamine can be distinguished by their reactions with the help of nitrous acid, which is prepared in situ from a mineral acid and sodium nitrite. Benzylamine reacts with nitrous acid to form unstable diazonium salt, which in turn gives alcohol with the evolution of nitrogen gas.

$$\underset{\mathrm{Benzylamine}}{\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{CH}{\phantom{A}}_2-\mathrm{NH}{\phantom{A}}_2}+\mathrm{HNO}{\phantom{A}}_2\stackrel{\mathrm{NaNO}{\phantom{A}}_2+\mathrm{HCl}}{\to }\underset{(\mathrm{Unstable})}{[\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{CH}{\phantom{A}}_2-\stackrel{+}{\mathrm{N}{\phantom{A}}_2}\mathrm{C}\stackrel{-}{\mathrm{l}}]}\stackrel{\mathrm{H}{\phantom{A}}_2\mathrm{O}}{\to }\mathrm{N}{\phantom{A}}_2\uparrow +\underset{\mathrm{Benzy}\mathrm{alcohol}}{\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{CH}{\phantom{A}}_2-\mathrm{OH}}+\mathrm{HCl}$$

On the other hand, aniline reacts with HNO₂ at a low temperature to form stable diazonium salt. Thus, nitrogen gas is not evolved.

$$\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{NH}{\phantom{A}}_2\underset{273-278\mathrm{K}}{\overset{\mathrm{NaNO}{\phantom{A}}_2+\mathrm{HCl}}{\to }}\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5-\stackrel{+}{\mathrm{N}{\phantom{A}}_2}\mathrm{C}\stackrel{-}{\mathrm{l}}+\mathrm{NaCl}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}$$